The invention relates to an unsaturated high-pressure sodium lamp of the type having a discharge vessel enclosing a discharge space, and discharge electrodes therein. At least one electrode is provided with emitter material and the discharge vessel contains a fill material comprising sodium, mercury and a rare gas. The invention further relates to a method of manufacturing such a lamp.
The term "ceramic wall" is to be understood herein to mean a wall formed by translucent crystalline metal oxide which may be either monocrystalline (for example sapphire) or polycrystalline. Known polycrystalline metal oxides in this respect are aluminium oxide and yttrium aluminium garnet. In polycrystalline form the material is sintered to gas-tightness. Such a lamp is known from U.S. Pat. No. 3,453,477.
In the known lamp formation of sodium aluminate is counteracted by a suitable choice of the temperature and the composition ratio of Na/Hg in the lamp fill material. However, experiments have shown that not only the metal oxide of the ceramic wall gives rise to the disappearance of Na from the fill material, but also that a number of oxygen sources give rise to the disappearance of sodium. A very important source is found to be emitter material, for instance in the case in which this material contains alkaline earth metal oxide, or for instance yttrium oxide. Thus, it has been found that the use of such an emitter material results in a substantial disappearance of sodium during the first hundred hours of lamp operation and in a small, but continuing disappearance thereof during further lamp operation. Although a number of metals are known to function as an oxygen getter in a lamp discharge space, these metals do not lead in the presence of emitter material to an acceptable suppression of the processes extracting sodium from the fill component and from the discharge space.
Although it is conceivable in principle to compensate the loss of Na occurring at the beginning of the lamp life by increasing the quantity of Na in the initial fill material, it is thus hardly possible in practical conditions to manufacture lamps on an industrial scale in a reproducible manner. Moreover, also in this case the lamp will exhibit a changing behavior during the first hours of its life.